Liquid crystal projectors are widely used to project images on a screen. The liquid projectors are classified into two types, a front-projection type to project images from the front side of a screen, and a rear-projection type to project images from the rear side of a screen.
Also, liquid crystal panels (liquid crystal cells) are classified into a transmission type and a reflection type, each of which is used in the liquid crystal projector. When projecting an image, the liquid crystal projector firstly generates information light by casting light onto a liquid crystal panel that displays an image, and projects the information light through a projection lens to form an image on a screen. However, the liquid crystal panels, which act on different liquid crystal modes, can only provide a relatively narrow field-of-view in each liquid crystal mode.
In a normally white TN (twisted nematic) liquid crystal panel, for example, linearly polarized rays that enter perpendicular to a liquid crystal layer with no voltage applied will rotate their polarization wave fronts by 90° along twist-aligned liquid crystal molecules. These linearly polarized rays pass through a polarizer allocated on a light exit side of the liquid crystal panel, and the TN liquid crystal panel appears white (white display state). When a voltage is applied to the liquid crystal layer, in contrast, the liquid crystal molecules are released from the twist alignment and linearly polarized rays entering perpendicular to the liquid crystal layer will pass through the liquid crystal layer without rotating their polarization wave fronts. These linearly polarized rays are blocked by the polarizer, and the TN liquid crystal panel appears black (black display state).
Even in the black display state, however, the liquid crystal layer offers birefringence to obliquely incident light. In other words, light rays obliquely entering the TN liquid crystal panel in the black display state have phase difference and are modulated into elliptically-polarized rays during the passage through the liquid crystal layer. The elliptically-polarized rays pass through the polarizer on the light exit side, and lower the density of black display, narrowing the field-of-view of the TN liquid crystal panel.
This problem results from the liquid crystal molecules lying near substrates that hold the liquid crystal layer. These liquid crystal molecules are not aligned perpendicular to the substrate surfaces completely even when a voltage is applied to the liquid crystal layer. Namely, in the vicinity of the substrates, the liquid crystal molecules are aligned such that more distant molecules from the substrates are more tilted to the substrate surfaces. These gradually-tilted liquid crystal molecules (hereinafter, tilt components) are birefringent to the light rays passing obliquely through the liquid crystal layer, whereas they exhibit little birefringence to the light rays passing orthogonally through the liquid crystal layer. As a result, light modulation performance of the TN liquid crystal panel depends on the angle-of-incidence of a light ray to the liquid crystal layer, and affects the density of black display. Note that, in the liquid crystal projectors, a light ray enters a pixel from a conical area with a cone angle of approximately 15° to a surface normal of the liquid crystal panel.
Such angle dependence is not only found in the TN liquid crystal panels, but also in the liquid crystal panels of other liquid crystal modes, such as VAN, OCB and ECB, so long as they contain the tilt components in the black display state.
As for direct-view-type liquid crystal display devices, on the other hand, contrast degradation problem due to the angle dependence can be eliminated by a phase compensator. For example, a phase compensator of this type has been marketed as “Fuji WV Film wide-view A” (product name/WV film) from Fujifilm Corporation. Also, a thin film of an obliquely deposited material on a base plate (hereinafter, oblique deposition film) can be used as a phase compensator. Having a birefringence property, the oblique deposition film is able to compensate the phase difference caused by the tilt components, and expand the field-of-view of the liquid crystal panels (see, for example, U.S. Pat. No. 5,638,197).
In the meanwhile, the phase compensators are also used in the liquid crystal projectors so as to improve contrast of a projection image. For example, there is a liquid crystal projector having a phase compensator made of inorganic material such as the aforesaid WV film (Japanese Patent Laid-open Publication No. 2002-14345). Another exemplary usage is a liquid crystal projector having a phase compensator made of discotic liquid crystal molecules solidified in hybrid orientation (Japanese Patent Laid-open Publication No. 2002-131750).
Exemplary usage of the phase compensator made of an inorganic material would be found in prior art, such as Japanese Patent Laid-open Publication No. 2002-31782 which discloses using a single crystal sapphire, quartz or such uniaxial birefringent substance as the inorganic material phase compensator, U.S. Pat. No. 5,196,953 which discloses a liquid crystal projector having a birefringence structure of inorganic thin-film stack, and Japanese Patent Laid-open Publication No. 2004-102200 which discloses a liquid crystal projector using a combination of several phase compensators of different inorganic materials. In addition, European Patent Application Publication No. 0179640 discloses a method for manufacturing an A-plate in which a material is obliquely deposited on a revolving base plate inside an evaporator.
In general, a birefringent characteristic is represented by an index ellipsoid defined by three principal refractive indices. All the aforesaid phase compensators function as an O-plate that has an index ellipsoid tilted to the surface of the liquid crystal panel, and provide good contrast of projection images for liquid crystal projectors. Additionally, inorganic material oblique deposition films are biaxial birefringence in most cases, and used as O-plates. These oblique deposition films are known to have three principal refractive indices of different amplitudes (see, “Structure-related Optical Properties of Thin Films” by H. Angus Macleod, J. Vac. Sci. Technol. A, Volume 4, No. 3, 1986, pp. 418-422). The largest and smallest principal refractive indices are tilted to the base plate surface.
A phase compensator, when made of an organic material, would easily discolor if exposed to UV-containing intense light for a longtime. Especially, in the liquid crystal projectors which use a higher intensity light source and hit higher temperature than the direct-view-type liquid crystal monitors, the organic material phase compensator would be as weak as to start discoloring in only 2,000-3,000 hours.
When made from a birefringent crystal such as the single crystal sapphire or quartz, a phase compensator becomes durable enough, but high-precision control is required for a cutting surface and thickness of the crystal, making the phase compensator too expensive to use for commercial products.
As mentioned above, the oblique deposition film of inorganic material is a biaxial birefringent component. Conventional oblique deposition films cannot completely compensate the phase difference due to the tilt components. In other words, in the conventional oblique deposition films, a slow axis of retardation, when viewed from a surface normal direction of the base plate (hereinafter, frontal retardation), is normally parallel to a plane including a direction of the oblique deposition and the surface normal of the base plate. By changing an angle of evaporation, it is possible to make the slow axis perpendicular to this plane, but the frontal retardation would still only have a small value. Therefore, if the conventional inorganic material oblique deposition film is used to compensate the phase difference caused by the tilt components, a retarder having an optic axis parallel to the surface of the liquid crystal panel or an A-plate has to be used in combination.
In view of the forgoing, an object of the present invention is to provide a durable, low cost and easily manufacturable biaxial brefringent component which by itself can properly compensate phase difference in liquid crystal panels.